1. Field of the Invention
This invention relates generally to single-stage mechanical-drive steam and gas-expansion turbines, and is concerned in particular with improvements in the bearings employed to rotatably support the rotor shafts of such turbines, as well as to improvements in the systems used to lubricate such bearings.
2. Description of the Prior Art
Historically, mechanical-drive turbines have been fitted either with antifriction bearings of the ball or roller type, or with hydrodynamically lubricated sliding metallic sleeve bearings. Antifriction bearings must be replaced periodically, with attendant interruptions in turbine operation. Because of such periodic service interruptions, the use of antifriction bearings is generally restricted to relatively low-powered, smaller turbines in non-critical service applications.
In contrast, if properly installed and lubricated, sleeve bearings have virtually unlimited life. Thus, sleeve bearings have been applied to all types of mechanical-drive turbines, including those with higher power ratings, and in particular those operating in critical service applications where process requirements cannot tolerate periodic outages to accommodate bearing maintenance.
Turbine sleeve bearings have conventionally comprised multiple metallic layers, with the innermost layer which bears against the rotor journal usually consisting of a babbitt material. The babbitt material is a relatively soft metal alloy of lead, tin, antimony or copper in various proportions. Because of their relative softness, babbitt materials are characterized by a property commonly referred to as "imbedability", i.e. an ability to absorb reasonable quantities and sizes of foreign contaminants such as metal particles and debris which become embedded in the babbitt material without resulting damage to the journal. The babbitt materials are also considered to have "conformability", which means that they tend to wear-in during initial service, thereby harmlessly accommodating minor imperfections and misaligmnents in the journals and bearings.
The conventional sleeve bearings are commonly lubricated hydrodynamically at low to moderate speeds by one or more oil rings which ride on the rotor journal through "windows" provided at the top of the bearing sleeve. The oil rings extend below the journal and sleeve and are partially submerged in a lubricant pool contained within the bearing housing. As the journal revolves, the oil ring also revolves as a result of its frictional contact with the journal surface, and in so doing the oil ring picks up lubricant by surface tension from the underlying pool. This oil is then deposited on the journal and is distributed by gravity and viscous effects into the sleeve bearing where it serves to support the rotating journal on a hydrodynamically created oil film.
Under ideal operating conditions, the conventional oil ring-type lubrication system operates in a generally satisfactory manner to provide adequate lubrication for the bearing. However, the higher operating pressures, temperatures, speeds and attendant structural deflections of contemporary turbines have sorely taxed the capabilities of conventional lubrication systems, often resulting in an insufficient delivery of lubricant to the bearing.
An inherent limitation in conventional sleeve bearing lubrication systems stems from the fact that oil ring rotary speed is not directly related to the rotational speed of the journal. More particularly, at increasing journal speeds, the oil ring increasingly exhibits a tendency to "slip" on the journal. It appears that such slippage results from the oil ring itself riding on an oil fill on the journal, rather than being directly driven by metal-to-metal traction between the ring and journal. This condition is further exacerbated by the increased force required to drag the oil ring through the underlying pool of oil. The net effect is that at increasing journal speeds, oil ring speeds gradually level off, and as a result the oil rings become incapable of continuing to supply enough oil to support proper lubrication and heat removal.
A further difficulty with the conventional oil ring arrangement is that oil must be delivered to the bearing from the inside diameter of the ring where it rides on the journal. However, at higher rotational speeds, oil is centrifuged out of the ring's inner surface and is thus cast off radially to the bearing housing walls rather than being delivered to the bearing where it is needed.
Insufficient bearing lubrication quickly translates into increased bearing temperatures. The conventional babbitt materials, because of their softness, are relatively weak and their strength diminishes progressively at increasing temperatures. When lubrication is marginal, or if the bearings are initially subjected to misalignment, they tend to rapidly overheat and fail. The first mode of this failure is commonly referred to as "wiping", a term that reflects the circumferential tearing and smearing of the distressed babbitt surface. If the source of distress is severe and not timely corrected, the deterioration may progress precipitously, with a resulting catastrophic failure of either or both the bearing and journal.